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Most reciprocating aircraft engines have two spark plugs per cylinder. What are the reasons for this?

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  • $\begingroup$ It's not only aircraft engines, the Alfa Romeo's Twin Spark engines have two spark plugs per cylinder as well - mainly for performance reasons. $\endgroup$ – Pavel Aug 5 '15 at 9:05
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Bold Method has a great post today about why aircraft engines have more than one spark plug per cylinder. There are a few basic reasons.

Reliability

Having two sparks plugs is more reliable. If one spark plug becomes inoperative for some reason, there's a second to provide the spark for the power stroke.

It should also be noted that the spark plugs are usually powered by dual, independent magnetos or, in some cases, an electronic capacitor discharge ignition. The redundancy of the ignition system adds to the reliability.

More Power

Having two spark plugs means the flame ignites from two points leading to more power per power stroke. This point is demonstrated during run-up. When the magneto check is done the engine RPMs drop when running on a single mag and set of spark plugs.

Even Combustion

Two sparks plugs and a two flame front leads to a more even burn of the fuel air mixture for a smoother running engine.

Prevent Fouling

Aircraft engines burn leaded fuel which can lead to getting lead deposits on the spark plugs. Having dual plugs leads to a more complete burn which can prevent the deposits from forming.

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    $\begingroup$ regarding reliability, a plane with dual plugs also has dual magnetos and therefore separate wiring, and the entire ignition system is redundant. So even if an entire mag fails, you still have redundancy. $\endgroup$ – rbp Aug 4 '15 at 15:15
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    $\begingroup$ Good point. Updated. $\endgroup$ – ryan1618 Aug 4 '15 at 15:43
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    $\begingroup$ en.wikipedia.org/wiki/Dual_ignition $\endgroup$ – Digital Trauma Aug 4 '15 at 19:56
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    $\begingroup$ Aircraft engines tend to have a fairly large cylinder capacity, hence a wide bore. The time the flame front takes to travel across this bore is significant. I presume the increased power is due to the most useful section of the fuel burn being biased to the time the crank / connecting rod is in the most advantageous position. Down side might be that you have a very rapidly increasing combustion chamber pressure (ie, you are burning the mixture significantly quicker with 2 flame fronts) possibly making detonation more likely. $\endgroup$ – Kickstart Aug 5 '15 at 14:25
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    $\begingroup$ @Sean - I suspect no further advantages, added to which adding more limits other parts of the design (such as valve position - easy enough to have 2 spark plugs of a 2 valve per cylinder engine). Aircraft engines tend to be low revving, so a reasonable amount of time for the mixture to burn, while more spark plugs = faster rise in cylinder pressure. $\endgroup$ – Kickstart Jun 13 at 8:00
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Engine Longevity as well. The more even combustion resulting from dual sparks prevents hot spots inside the cylinder, resulting in improved engine life.

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Reliability is the key thing. The ignition system of a gasoline engine (the "magnetos" in most aircraft) is absolutely critical to the engine's operation, and thus the safety of the flight of the aircraft possessing it. A small break in a wire, or fouling of one plug, could cause cylinder misfires or even a total engine failure. To guard against this, dual independent ignition systems are incorporated into aviation engines to allow for continued operation in the event that any component of one ignition system fails.

Efficiency is a secondary benefit. Two spark plugs per cylinder means the spark initiates combustion at two points within the volume of the cylinder. Combustion rate of a vaporized fuel is typically a function of "surface area" of the region of combustion, so two sparks equals double the combustion surface area. This produces faster and more complete combustion, increasing power at a given fuel flow rate. One of the rudimentary "runup" tests of a piston engine involves selecting one or the other magneto, and watching the performance of the engine. The normal result with a healthy ignition system is a slight reduction in power, as indicated (in a fixed-pitch prop at least) by lower RPM. A severe power reduction, rough operation or an engine quit is a sign something's wrong on that side of the ignition system.

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  • $\begingroup$ How would a failure of one sparkplug or the breakage of one sparkplug wire be capable of causing a total engine failure? One would expect, at the very worst, that particular cylinder to stop producing power, but the rest of the engine to keep burning happily. $\endgroup$ – Sean Jun 12 at 23:03
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    $\begingroup$ @Sean - depends on which wire breaks, as he didn't mentioned sparkplug wires. One inside the magneto could easily stop all sparks on that system. And in certain circumstances it would be possible for a failed spark to result in enough fuel accumulating in a cylinder rapidly to hydraulic the engine (this can be an issue with high performance engines running on methanol) $\endgroup$ – Kickstart Jun 13 at 8:06

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